In studies designed to determine reproductive hormone responses of male mice following acute exposure to simulated high altitude (hypoxia), it was found that moderate hypoxia significantly reduced plasma concentrations of luteinizing hormone (LH), and to a lesser degree follicle stimulating hormone (FSH), and growth hormone (GH), even in the absence of changes in dietary intake, systemic hypothermia and the stress related rise in plasma cortiocosterone concontration (B). Prolonged exposure of mice to moderate to severe degrees of hypoxia depressed plasma LH concentration for 1 day, and plasma FSH and corticosterone for as long as 3 weeks. In studies designed to determine the altitude tolerance of male rats ranging from 30 to 600 days of age it was found that the critical threshold for survival in immature rats was 8230 m, and for young adult and old rats 7620 m. In immature rats the degree of hypothermia and elevated plasma B concentration were the most reliable criteria for determining altitude tolerance, and in young adult and old rats, changes in plasma aspartate amino transferase activity was the most important criterion for predicting altitude tolerance.